1. Field of the Invention
Embodiments of the invention generally relate to a method and apparatus for metering catalyst in a fluid catalytic cracking catalyst injection system and the like.
2. Description of the Related Art
FIG. 1 is a simplified schematic of one embodiment of a conventional fluid catalytic cracking system 130. The fluid catalytic cracking system 130 includes a fluid catalytic cracking (FCC) unit 110 coupled to a catalyst injection system 100, an oil feed stock source 104, an exhaust system 114 and a distillation system 116. One or more catalysts from the catalyst injection system 100 and oil from the oil feed stock source 104 are delivered to the FCC unit 110. The oil and catalysts are combined to produce an oil vapor that is collected and separated into various petrochemical products in the distillation system 116. The exhaust system 114 is coupled to the FCC unit 110 and is adapted to control and/or monitor the exhausted byproducts of the fluid cracking process.
The catalyst injection system 100 may include a main catalyst injector 102 and one or more additive injectors 106. The main catalyst injector 102 and the additive injector 106 are coupled to the FCC unit 110 by a process line 122. A fluid source, such as a blower or air compressor 108, is coupled to the process line 122 and provides pressurized fluid, such as air, that is utilized to carry the various powdered catalysts from the injectors 102, 106 through the process line 122 where they are combined with oil from the oil feed stock source 104 and delivered into the FCC unit 110.
FIG. 2 is one embodiment of a conventional additive injector 106. The additive injector 106 includes a pressure vessel 220 and a low pressure storage vessel 240. The pressure vessel 220 is coupled to one or more load cells 210 for weighing the catalyst that will be introduced into the FCC unit 110 through the process line 122. In operation, the catalyst is dispensed into the pressure vessel 220 at atmospheric pressure from the low pressure storage vessel 240. The pressure vessel 220 is subsequently weighed to determine the amount of catalyst loaded therein. The pressure vessel 220 is then pressurized by a pressure control device 228 coupled to the vessel 220 to a level that facilitates movement of the pressurized catalyst into process line 122 and then into the FCC unit 110. If the pressure vessel 220 is supported by any of the structural components surrounding it, other than the load cells 210 (such as pipes, electrical conduits, and the like), those components will prevent the load cells 210 from accurately measuring the weight of catalyst added to the pressure vessel 220, and ultimately into the FCC unit 100. Therefore, in order to obtain a reasonably accurate measure of the catalyst, the pressure vessel 220 must not be supported by other components of the system.
To isolate the pressure vessel 220 from the components coupled thereto, flexible connectors, such as bellows 230, are used to couple the pressure vessel 220 to the low pressure vessel 240, the process line 122, and other surrounding components. The bellows 230 allow the pressure vessel 220 to “float” on the load cells 210 so a more accurate reading may be obtained. However, use of flexible bellows 230 does not reliably insure accurate weight measurement of the pressure vessel 220. For example, the weight of the pressure vessel 220 is still slightly supported by the flexible bellows 230—a problem compounded by the fact that a plurality of bellows 230 must be utilized to isolate the pressure vessel 220 from the various components coupled thereto. Therefore, the determination of the weight of the catalyst added to the pressure vessel 220 is still not accurate. Moreover, due to the operating pressures and potentially explosive atmosphere, bellows meeting operational standards are quite expensive and wear quickly, resulting in the drift of weight readings, catalyst dust leaks and associated environmental issues, as well as necessitating costly process downtime and bellows replacement.
FIG. 3 is another embodiment of an additive injector 300. The injector 300 includes a high pressure storage vessel 340 coupled by a metering valve 330 to the process line 122. The metering valve 330 may be actuated to allow a predefined amount of catalyst to be introduced into the process line 122 and combine with the oil from the oil feed stock source 104 before entering the FCC unit 110. The high pressure storage vessel 340 contains a bulk supply of catalyst, for example, from about 1 to about 20 tons of catalyst, and is maintained at a pressure between about 50 to about 60 pounds per square inch (psi) by a pressure control device 320. As such, the pressure vessel 340 is subject to regulatory construction standards which cause the vessel to be relatively expensive as compared to a comparably sized, low pressure storage vessel. The high pressure vessel 340 is coupled to a plurality of load cells 310 which enable the weight of the high pressure storage vessel 340 to be determined. The weight of the catalyst injected is determined by comparing the weight of the high pressure storage vessel 340 before and after catalyst injection.
Metering catalyst in the manner described with reference to FIG. 3 eliminates the need for bellows used to isolate the pressure vessel. However, large high pressure storage vessels are very expensive. Therefore, there is a need for a method and apparatus for metering catalyst in a fluid catalytic cracking catalyst injection system that minimizes the cost of ownership.